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United States Patent |
6,207,023
|
Cha
|
March 27, 2001
|
Process for microwave air purification
Abstract
A process of air purification occurs in the presence of activated carbon or
its equivalent by decomposing adsorbed hazardous materials, such as
volatile organic compounds, on the carbon surface by radiofrequency energy
in the microwave range at near ambient conditions of temperature and
pressure. Further microwave oxidation to nonhazardous gases occurs in the
presence of an oxidation catalyst.
Inventors:
|
Cha; Chang Yul (3807 Reynolds St., Laramie, WY 82072)
|
Appl. No.:
|
249966 |
Filed:
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February 12, 1999 |
Current U.S. Class: |
204/157.3; 204/157.43; 204/158.2 |
Intern'l Class: |
B01D 53//00 |
Field of Search: |
204/157.3,157.43,158.2
96/223
422/21
219/678,679
|
References Cited
U.S. Patent Documents
4144189 | Mar., 1979 | Kirkbride | 252/414.
|
4968403 | Nov., 1990 | Herbst et al. | 208/113.
|
5227598 | Jul., 1993 | Woodmansee et al. | 219/10.
|
5268343 | Dec., 1993 | Hopp et al. | 502/5.
|
5269892 | Dec., 1993 | Cha | 204/157.
|
5367147 | Nov., 1994 | Kim et al. | 219/698.
|
Other References
Kirk-Othmer, Encyclopedia of Chemical Technology, 3rd Ed., Supp. Vol.,
"Plasma Technology," 599-608, John Wiley, NY--no date available.
Kirk-Othmer, Encyclopedia of Chemical Technology,3rd. Ed., vol. 15,
"Microwave Technology," 494-522, John Wiley, NY--no date available.
|
Primary Examiner: Wong; Edna
Attorney, Agent or Firm: Mingle; John O.
Claims
I claim:
1. A process for microwave purification of a gaseous stream with
nonhazardous releases comprising:
adsorbing impurities from said gaseous stream with an unsaturated bed of
carbonaceous material until a substantially saturated bed occurs while
producing a clean release gas;
irradiating said substantially saturated bed with microwaves while a
simultaneous sweep gas removes desorbed vapors;
passing said sweep gas through an oxidation catalyst bed irradiated by
microwaves, wherein said oxidation catalyst is deposited upon a microwave
absorbing substrate, and wherein sufficient oxygen is added to obtain
substantially complete oxidation of remaining vapors;
passing said sweep gas through an alkaline solution scrubber to remove
halide acids; and
releasing the non-hazardous sweep gas.
2. The process according to claim 1 wherein said halide acids further
comprise decomposition products from chlorinated organics.
3. The process according to claim 1 wherein said alkaline solution further
comprises sodium hydroxide solution.
4. The process according to claim 1 wherein said halide acids further
comprise hydrogen chloride.
5. The process according to claim 1 wherein said impurities further
comprise hazardous matter.
6. The process according to claim 1 wherein said impurities further
comprise volatile organic compounds.
7. The process according to claim 1 wherein said sweep gas further
comprises environmentally acceptable gaseous compounds.
8. The process according to claim 1 wherein said sweep gas further
comprises being selected from the group consisting of nitrogen, helium,
and carbon dioxide.
9. The process according to claim 1 wherein said microwaves further
comprise radiofrequency energy selected from the frequency range
consisting of 500 to 5000 Mhz.
10. The process according to claim 1 wherein said carbonaceous material
further comprises being selected from the group consisting of activated
carbon, char, soot, pyrolytic carbon, carbon black, activated charcoal,
and metal carbides.
11. The process according to claim 1 wherein said oxidation catalyst bed
further comprises being selected from the group of beds consisting of
fluidized, fixed, semi-fluidized, suspended, and moving.
12. The process according to claim 1 wherein said bed of carbonaceous
material further comprises being selected from the group of beds
consisting of fluidized, fixed, semi-fluidized, suspended, and moving.
13. The process according to claim 1 wherein said microwave absorbing
substrate further comprises impregnation with metal carbides.
14. The process according to claim 1 wherein said releasing the
non-hazardous sweep gas further comprises recycling said sweep gas.
15. A process for microwave air purification with environmentally
acceptable releases comprising:
adsorbing impurities from said air with an unsaturated bed of activated
carbon until a substantially saturated bed occurs while producing clean
air;
irradiating said substantially saturated bed with microwaves while a
simultaneous sweep gas removes desorbed vapors, where said sweep gas is
selected from the group consisting of nitrogen, helium, and carbon
dioxide;
passing said sweep gas through an oxidation catalyst bed irradiated by
microwaves, wherein said oxidation catalyst is deposited upon a substrate
impregnated with silicon carbide, and wherein sufficient oxygen is added
to obtain substantially complete oxidation of remaining vapors;
passing said sweep gas through an alkaline solution scrubber to remove
halide acids, and
releasing the environmentally acceptable sweep gas.
16. The process according to claim 15 wherein all beds further comprise
being selected from the group of beds consisting of fluidized, fixed,
semi-fluidized, suspended, and moving.
17. The process according to claim 15 wherein said microwaves further
comprise radiofrequency energy selected from the frequency range
consisting of 500 to 5000 Mhz.
18. A process for microwave halide-free air purification with
environmentally acceptable releases comprising:
adsorbing impurities from said air with an unsaturated bed of activated
carbon until a substantially saturated bed occurs while producing clean
air;
irradiating said substantially saturated bed with microwaves while a
simultaneous sweep gas removes desorbed vapors, where said sweep gas is
selected from the group consisting of nitrogen, helium, and carbon
dioxide;
passing said sweep gas through an oxidation catalyst bed irradiated by
microwaves, wherein said oxidation catalyst is deposited upon a substrate
impregnated with silicon carbide, and wherein sufficient oxygen is added
to obtain substantially complete oxidation of remaining vapors; and
releasing the environmentally acceptable sweep gas.
19. The process according to claim 18 wherein all beds further comprise
being selected from the group of beds consisting of fluidized, fixed,
semi-fluidized, suspended, and moving.
20. The process according to claim 18 wherein said microwaves further
comprise radiofrequency energy selected from the frequency range
consisting of 500 to 5000 Mhz.
Description
BACKGROUND OF INVENTION
1. Field of Invention
The present invention relates to a process using radiofrequency microwave
energy to purify an air stream containing hazardous materials,
particularly organic vapors, using carbonaceous adsorption followed by
sweep gas microwave desorption with subsequent environmental cleanup.
2. Background
Hazardous waste material often occurs which contains organic compounds that
are easily volatilized under common conditions. These sometimes naturally
pollute a gas stream, like air, and sometimes result in polluted solids,
like soil. In all instances release is not allowed without conversion to
environmentally clean gases.
Such organic compounds, often called just organics, cover a wide variety
and often contain halogen atoms, particularly chlorine, which originated
from various previously employed solvents. For instance, a contaminated
soil area which over the years had various liquid discharges containing
organics dumped on it was tested and a wide range of concentrations of
hazardous compounds detected. These are treated as a group to convert them
into environmentally releasable compounds of water and carbon dioxide;
however, to perform this the halide compounds are separately treated. The
most typical halogen atoms end up as gaseous halide acids so they are
commonly scrubbed with alkali and removed.
Adsorption of organics occurs readily upon carbonaceous materials, such as
activated carbon. Thus a contaminated air stream passed through a bed of
activated carbon will substantially purify it. Saturation of the bed will
eventually occur so removal of the adsorbed organics is performed to allow
recycling of the activated carbon. This desorption is conventionally
performed by heating the bed to volatilize the organics. For instance,
conventionally steam is employed for this task.
The subject invention employs microwaves for this desorption since
activated carbon is a very good absorber of such microwaves. Then the
desorbed volatiles, which are not necessarily in the same chemical form as
they were when adsorption occurred, are collected by a sweep gas which is
then treated to purify it using microwaves before release.
Quantum radiofrequency (RF) physics is based upon the phenomenon of
resonant interaction with matter of electromagnetic radiation in the
microwave and RF regions since every atom or molecule can absorb, and thus
radiate, electromagnetic waves of various wavelengths. The rotational and
vibrational frequencies of the electrons represent the most important
frequency range. The electromagnetic frequency spectrum is usually divided
into ultrasonic, microwave, and optical regions. The microwave region is
from 300 megahertz (MHz) to 300 gigahertz (GHz) and encompasses
frequencies used for much communication equipment. For instance, refer to
Cook, Microwave Principles and Systems, Prentice-Hall, 1986.
Often the term microwaves or microwave energy is applied to a broad range
of radiofrequency energies particularly with respect to the common heating
frequencies, 915 MHz and 2450 MHz. The former is often employed in
industrial heating applications while the latter is the frequency of the
common household microwave oven and therefore represents a good frequency
to excite water molecules. In this writing the term "microwaves" is
generally employed to represent "radiofrequency energies selected from the
range of about 500 to 5000 MHz", since in a practical sense this total
range is employable for the subject invention.
The absorption of microwaves by the energy bands, particularly the
vibrational energy levels, of atoms or molecules results in the thermal
activation of the nonplasma material and the excitation of valence
electrons. The nonplasma nature of these interactions is important for a
separate and distinct form of heating employs plasma formed by arc
conditions at a high temperature, often more than 3000.degree. F., and at
much reduced pressures or vacuum conditions. For instance, refer to
Kirk-Othmer, Encyclopedia of Chemical Technology, 3rd Edition,
Supplementary Volume, pages 599-608, Plasma Technology. In microwave
technology, as applied in the subject invention, neither condition is
present and therefore no plasmas are formed.
Microwaves lower the effective activation energy required for desirable
chemical reactions since they can act locally on a microscopic scale by
exciting electrons of a group of specific atoms in contrast to normal
global heating which raises the bulk temperature. Further this microscopic
interaction is favored by polar molecules whose electrons become easily
locally excited leading to high chemical activity; however, nonpolar
molecules adjacent to such polar molecules are also affected but at a
reduced extent. An example is the heating of polar water molecules in a
common household microwave oven where the container is of nonpolar
material, that is, microwave-passing, and stays relatively cool.
In this sense microwaves are often referred to as a form of catalysis when
applied to chemical reaction rates. For instance, refer to Kirk-Othmer,
Encyclopedia of Chemical Technology, 3rd Edition, Volume 15, pages
494-517, Microwave Technology.
Related U.S. microwave patents include:
No. Inventor Year
4,144,189 Kirkbride 1979
4,968,403 Herbst et al. 1990
5,269,892 Cha 1993
5,268,343 Hopp et al. 1993
Referring to the above list, Kirkbride discloses regeneration of spent
fluid cracking catalysts by heating with microwaves to a range of
700-900.degree. F. to remove coke; however, preheating by conventional
means is suggested before usage of microwaves. The subject invention
operates with much lower temperatures by microwave catalysis not just
microwave heating.
Herbst et al. discloses an improvement in the regeneration of cracking
catalysts by selective use of microwave heating. High temperatures in the
650-750.degree. C. range are employed. The subject invention employs
microwave catalysis not just microwave heating.
Cha discloses char-gas oxide reactions, such as NO.sub.x decomposition,
catalyzed by microwaves, but does not decompose general hazardous matter,
like organics. Yet this shows that if any NO.sub.x was present, it is made
environmentally safe.
Hopp et al. disclose a conventional reactivation process for activated
charcoal catalyst used with the preparation of R-227 refrigerant by
heating to the 450-900.degree. C. range. No microwaves are employed. The
subject invention operates with much lower temperatures by microwave
catalysis.
SUMMARY OF INVENTION
The objectives of the present invention include overcoming the
above-mentioned deficiencies in the prior art and providing a potentially
economically viable process for the microwave cleanup of contaminated air.
This process occurs in the presence of activated carbon or its equivalent
by desorbing and decomposing adsorbed hazardous materials on the carbon
surface by radiofrequency energy in the microwave range at near ambient
conditions of temperature and pressure. A further oxidation to
nonhazardous gases occurs in the presence of an oxidation catalyst also
employing microwaves.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a flow process for the removal of organics from a air stream
and subsequent environmental cleanup.
FIG. 2 shows an experimental apparatus for activated carbon regeneration.
DETAILED DESCRIPTION OF INVENTION
Microwaves are a versatile form of energy that is applicable to enhance
chemical reactions since the energy is locally applied by its largely
vibrational absorption by nonpolar molecules and does not produce plasma
conditions. Particularly reactions that proceed by free-radical mechanisms
are often enhanced to higher rates because their initial equilibrium
thermodynamics is unfavorable. A second class of enhanced reactions are
those whose reaction kinetics appear unfavorable at desirable bulk
temperature conditions.
Carbonaceous material is an excellent microwaves absorber since it has a
wide range of polar impurities that readily interact with such
radiofrequency energy especially in electron vibrational modes.
Consequently the microwave waveguide design for the microwave cavity is
not usually critical. Carbonaceous material for use with the subject
invention commonly comprises activated carbon, char, soot, pyrolytic
carbon, carbon black, activated charcoal, and metal carbides. In many
instances activated carbon is the preferred material to employ under
ambient temperature and pressure conditions, although activated charcoal,
if readily available, is likely more cost effective. However in gaseous
systems, especially at higher temperatures, other carbonaceous materials
such as metal carbides, especially silicon carbide, are convenient to
utilize. Silicon carbide is conveniently utilized as a microwave absorbing
substrate to enhance conventional catalytic processes.
The microwave excitation of the molecules of the carbonaceous material,
often referred to as microwave catalysis, excites constituents, such as
hazardous matter and contaminants including organics, which have been
adsorbed on the internal pore surfaces of the carbonaceous material and
produces a highly reactive condition. Further molecules from the carrier
medium, such as a sweep gas, are in close proximity or within the surface
boundary layer of the carbon surface through chemisorption, absorption,
adsorption, or diffusion, and additional chemical reactions with these
constituents may occur.
An equimolar mixture of toluene, o-xylene, and trichloroethylene was
employed as a typical combination of volatile organic compounds (VOC).
The desorption process potentially produces a wide range of chemical
compounds since the microwave excited carbon surface and possibly the
sweep gas molecules react with various decomposition products from the
adsorbed constituents. The required final step consists of the microwave
enhanced oxidation of hydrocarbons into water and carbon dioxide to
produce an environmentally acceptable discharge. Additionally if halogens,
such as chlorine, are present in the VOC, then gaseous halide acids, the
most common which is hydrogen chloride, will form and removal by sodium
hydroxide scrubbing occurs.
FIG. 1 shows a process flow sheet for microwave air purification. All
components are conventional except those utilizing microwaves.
Contaminated air 20 enters and is pumped into the lower level 21 of a
moving bed 50 of carbonaceous material 51 fed from a lock-hopper 52 of new
or regenerated material and such material leaving into a receiving
lock-hopper 53. The air leaves the carbonaceous material bed 50 at the top
22 as clean air and discharged 23. In an air purification process with a
closed system such discharged air 23 is recycled. The saturated
carbonaceous material is transported 54 into a lock-hopper system 55 which
utilizes regeneration reactor off-gas 60 exhausted from the many
lock-hoppers and cleaned with a cyclone 56 before use as a recirculation
medium 65 to move the carbonaceous material around between various
lock-hoppers. The saturated carbonaceous material is fed as a moving bed
into the regeneration reactor 57 powered by a microwave system 58
utilizing a helix waveguide 59 and with cooling water in 61 and out 62.
After leaving the regeneration reactor 57, the regenerated carbonaceous
material is stored in a lock-hopper 63 and eventually recycled 64 back to
the main adsorption reactor feed lock-hopper 52. The sweep gas 40 enters
and passes down through the regeneration reactor 57 and leaves 41 and
flows into a standard water condenser 30 to collect any condensed liquid
and then flows 42 entering the oxidation reactor 31 where a stream 43
containing oxygen is mixed. The oxidation reactor 31 composed of a bed of
oxidation catalysts 32 is powered by a microwave system 34 connected to an
external microwave generator utilizing a helix waveguide 33 and with
cooling water in 35 and out 36. These oxidation catalysts are deposited
upon a substrate containing microwave absorbing carbonaceous material,
often silicon carbide. The gas 44 leaving the oxidation reactor 31 is then
tested for halide concentration, or alternatively the entering gas 20 is
tested for halide atoms, and if halides are not present, this gas is mixed
with the clean air 22 from the adsorption reactor 50 and discharged 23. If
halides are present, the gas 44 passes through a conventional alkali
scrubber system 37 which removes any halide acids and flows 45 before
being mixed with the clean gas 22 from the adsorption reactor 50 and then
discharged 23. It is to noted that the amount of flow 46 coming from the
cleanup system is only a few percent of the principal flow system 22
coming from the adsorption reactor 50 so a large dilution factor occurs.
The flow sheet shown in FIG. 1 contains many conventional accessories,
like pumps, valves, pressure gages, filters, etc., which are necessary for
safe operation of such a chemical process but are outside of the necessary
components of the subject invention.
FIG. 2 represents an experimental test apparatus to show the effective
microwave regeneration of trichloroethylene saturated activated carbon. A
quartz tube 79 is packed with activated carbon 85. Around the quartz tube
79 a helix 81 is wound as a microwave waveguide which is fed from
microwave connectors 84 which are cooled by entering water 82 and leaving
water 83 and are connected to an external microwave generator. Bleed gas
enters 80 and leaves 86 into a conventional water condenser 78 which
condenses liquid in the flask 77. The bleed gas then exits 76 the
experimental apparatus. With the microwave system off, bleed gas
containing trichloroethylene passes through the bed and is adsorbed until
saturation of the activated carbon occurs. Then trichloroethylene free
bleed gas is utilized using the microwave generator to regenerate the
activated carbon.
EXAMPLE 1
To study the stability of carbonaceous material under continuing adsorption
and desorption cycles, activated carbon was exposed to twenty such cycles.
The apparatus of FIG. 2 was employed with a VOC of 200 ppm
trichloroethylene. Nitrogen at 50 SCFH was the bleed gas. The microwave
energy employed was 850 watts. By the fourth cycle the adsorption capacity
had settled down into a substantially constant value appropriate to the
accuracy of the measurements of 40 grams trichloroethylene per 100 grams
of activated carbon. Thus recycling between many adsorption and desorption
cycles does not substantially degrade the activated carbon bed.
EXAMPLE 2
The oxidation catalyst apparatus, as shown in FIG. 1, was utilized in an
experimental setup to determine the efficiency of the oxidation step.
Conventional platinum or palladium oxidation catalysts, such as PRO-VOC-7
manufactured by Protech Company, or equivalent, was utilized on an alumina
substrate impregnated with approximately one-fourth by weight fine silicon
carbon particles. The input air stream contained a solvent composed of an
equimolar mixture of toluene, o-xylene, and trichloroethylene. The
microwave power was 850 watts. With a six inch deep oxidation catalyst bed
and a gas flow rate of 130 SCFH, solvent flow rates of from 30 to 70
mL/min produced substantially 100 percent destruction efficiency.
A process for microwave air purification with nonhazardous releases
comprising adsorbing impurities, such as hazardous materials like
organics, from said air with an unsaturated bed of carbonaceous material,
being selected from the group consisting of activated carbon, char, soot,
pyrolytic carbon, carbon black, activated charcoal, and metal carbides,
until a substantially saturated bed occurs. A clean overhead gas is
produced. To regenerate said substantially saturated bed of carbonaceous
material so recycling of it occurs, microwaves are employed with a
simultaneous sweep gas which removes desorbed vapors. Such sweep gas then
passes through an oxidation catalyst bed also irradiated by microwaves,
wherein said oxidation catalyst is deposited upon a microwave absorbing
substrate impregnated with silicon carbide, and wherein sufficient oxygen
is added to obtain substantially complete oxidation of the remaining
vapors. The sweep gas further passes through an alkaline solution
scrubber, such as sodium hydroxide, to remove any halide acids, such as
hydrogen chloride. In the situation where halides are known to be absent,
the scrubber step is omitted. Finally the non-hazardous, environmentally
acceptable sweep gas is released.
The sweep gas is selected from the group consisting of nitrogen, helium,
and carbon dioxide which represent environmentally neutral gases. The
microwaves are radiofrequency energy selected from the frequency range
consisting of 500 to 5000 Mhz. All beds, absorption, regeneration and
catalyst, are selected from the group of beds consisting of fluidized,
fixed, semi-fluidized, suspended, and moving.
The foregoing description of the specific embodiments will so fully reveal
the general nature of the invention that others can, by applying current
knowledge, readily modify and/or adapt for various applications such
specific embodiments without departing from the generic concept, and
therefore such adaptations or modifications are intended to be
comprehended within the meaning and range of equivalents of the disclosed
embodiments. It is to be understood that the phraseology or terminology
herein is for the purpose of description and not of limitation.
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